This invention relates to gas sensors, and, more particularly, to oxygen sensors that inhibit glass formation thereon.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. In automotive applications, the direct relationship between the oxygen concentration in the exhaust gas and the air-to-fuel ratio of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically comprises an ionically conductive solid electrolyte material, a porous electrode on the exterior surface of the electrolyte exposed to the exhaust gases with a porous protective overcoat, and an electrode on the interior surface of the sensor exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with platinum electrodes, which operate in potentiometric mode to detect the relative amounts of oxygen present in the exhaust of an automobile engine. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          xe2x80x83        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
pO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture.
In automotive applications employing oxygen sensors, data gathered by sensors is quantified and used to adjust the air-to-fuel ratio of the fuel mixture as it is fed in the engine. However, the performance of an oxygen sensor can be altered by the presence of poisons, which are generally the reaction products of components of the exhaust gas of the automotive system. These poisons typically result from the absorption of volatiles in the engine oil that either proceed directly into the combustion process or are revolatilized when the engine approaches its operating temperature. The poison vapors are drawn into the combustion chamber of the engine through the Positive Crankcase Ventilation (PCV) system and are then oxidized or reacted to form a smoke that contains molecule-sized particles. These particles travel in the exhaust gas and deposit on the surface of the oxygen sensor.
One type of poison is the deposition of impervious glass materials on the sensor surface, which often significantly impairs the reliability of the sensor element. These poisons form dense glass phases on the surface of the sensor and inhibit sensor performance. Typically, only a limiting amount of metallic particulates are present in the exhaust stream, and CaO and ZnO2 react with phosphates in the exhaust stream to form dense glass phases of CaPO4 and Zn(PO4)2. The formation of dense glass phases, which are deposited onto the surface of the sensor element, cause the time for particles to diffuse through the sensor element to increase significantly. This increase in diffusion time, in turn, causes NOx emissions to increase significantly, and causes a delay in the response of the system in adjusting the air-to-fuel ratio of the fuel mixture as it is fed to the engine.
What is needed in the art is a sensor element that inhibits the formation of dense glass phases of CaPO4 and Zn3(PO4)2.
The drawbacks and disadvantages of the prior art are overcome by the gas sensor element and method for its use. The gas sensor element, comprises: an electrochemical cell, a protective material in fluid communication with the electrochemical cell, and an inhibitive coating. The inhibitive coating, which is disposed on a side of the protective material opposite the electrochemical cell, comprises an alkaline earth material.
The method of operating the gas sensor element, comprises: exposing the gas sensor element to an exhaust gas. The exhaust gas is diffused through the inhibitive coating to the electrochemical cell where a constituent of the gas is sensed. Additionally, acid gases in the exhaust gas are reacted with the inhibitive coating.
The above-described and other features and advantages gas sensor and method for its use will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.